Giant screen projection delivers immersive experience people cannot get from other ordinary displays. Dome type of projection provides even larger field of view and a virtual reality feeling. But all the craving for visual dimension expansion comes with a squarely increased requirement of projector brightness. No matter for the latest Xeon based traditional DLP projector or the upcoming powerful laser light engine based projectors, stacking two or more projectors can always be a simple, efficient and cheaper way to achieve a brighter projection.
Stacking projection is overlapping two or more projectors' images substantially onto a common display surface. In a traditional stacked projection for 2D display, each projector displays an exact same image; and the stacking occurs only in one dimension which is normal to the display surface. The substantial part of each projected image is added up to produce a desired final image. The final image can have increased brightness and largely reduced screen door effect of a given native projector resolution, but it also requires very accurate sub-pixel inter-projector registration and significant effort for maintaining such registration to achieve the comparable image sharpness and quality as that from a single projector setup.
Registration is normally achieved by warping one image to match another image. During the warping, the pixels are digitally resampled from the original image by using a digital filter. When and in most of time, the new pixel is sampled between adjacent non-fractional pixels, based on Nyquist-Shannon sampling theorem, the maximum frequency information can be possibly represented in the resampled image is the half of the maximum discreet sampling frequency of the original image; that is, if there is a sharp one-pixel width line in original image, now it becomes a two-pixel blurred line in the warped image. So registration by warping will generally degrade image quality. Considering the complete variety of the image content, even with a perfect alignment mapping knowledge, the best possible image registration can be achieved by digital warping is 0.5 pixels.
In practice, the above theoretical best possible registration is impossible to get. Because alignment mapping knowledge is imperfect. (1) first, the registration and its calibration accuracy would suffer from system and measurement errors. A common way of registration calibration for a stacking system is using a camera system to capture geometry locations of a set of control points. Then calculate the geometrical mapping between control points in image and that on screen. The common error of this process arises from camera noise and its lower capturing resolution; and other errors are seen from the interpolation based estimation of mappings for other non-control point pixels. (2) Second, thermal drifting and aging arises from optical components, light engine components, electronic and mechanical parts may also cause registration calibration shift continuously and constantly over time. The drifting and aging is very non-uniform and unpredictable. Besides the common way of calibration is a separate process that needs a set of stricter testing conditions, e.g. complete dark without stray lights, and exclusive access of the complete projection system for a period of time; for example, it takes minutes to display a sequence of specially designed test pattern images. This means a calibration is inappropriate to be executed during a show or even more than once during a day. Therefore the registration changes are not captured and followed by the projection system. The larger registration errors can be seen as result of degraded images at the end of day. (3) Lastly the thermal, mechanical and environmental factors caused system component vibration makes very accurate registration almost impossible. For example, each projector may be affected by exhaust air or vent fan and sub-woofer from audio system differently, therefore may vibrate at different magnitudes, frequencies and phases. The relative position of every pixel from each projector becomes almost unpredictable. A vibration pattern was observed and measured in a typical 2K system could be up to 4 pixels within a second.
For 4K and higher resolution digital cinema projection, the above mentioned three kinds of difficulties for achieving very accurate registration become even more difficult if it not impossible. A 4K or higher resolution system is at least 4 times finer pixel size than 2K system; this requires even higher calibration measurement accuracy. Registration drifting and component aging caused image quality degradation would be much more sensitive. And the projector vibration would affect more pixels if other factors remain the same. If these problems can not be solved properly for a stacked projection, migrating to 4K or higher resolution projector is pointless and a waste of effort because it will not deliver the improved image quality as expected.
Even for other low end projection system, like home theaters, office presentation, etc. consumer applications, the resolution is 2K or less, but the calibration process needs to have less stringent requirements; hardware aging and drifting could be larger; there could be more factors in environment cause unpredictable vibration or change. Those also make alignment and maintaining alignment of a low end 2K or less resolution system difficult.
Besides registration, brightness uniformity is also a common problem of stacking projection but seldom being addressed. (1) Non-uniform brightness may arise between projectors, that is, one projector has different brightness or color than the other projector. This imposes a problem of inconsistent or incorrect color for even a traditional stacked projection which each projector displays the exactly same image. The pixel color in the final super-imposed image becomes a function of brightness of each color component of each projector. That is, for any two pixels having same color (hue) but different luminance in the original image or if two projectors have same brightness, now they are having different color (hue) because two projectors have different brightness. (2) Non-uniform brightness may also occur within one projector, that is, within single projected image, the peak brightness for each pixel is not uniform and following a distribution; and even worse, this distribution is changing frequently over time due to the same reasons of previous registration drifting from thermal uneven, light engine aging and optical mechanical movement. This kind of non-uniformity cause even more image degradation by introducing in-image vignetting, color shifting and inconsistent contrast/brightness.
To address all those problems mentioned above for a general stacking projection, a new method and efficient system is in need to be invented. The new method and system should not only be able to achieve fast, high accuracy brightness and registration calibration and correction, but also can easily, continuously and constantly monitor, update and maintain the existing calibration at low cost. The new method and system should also be largely immune to or tolerate most projection system random vibration and calibration errors.